This invention relates to equipment for weighing heavy loads and more particularly to tank weighing assemblies using load cells.
Various configurations of load cells and mounting hardware have been used in weighing assemblies for tanks. In one type of assembly a shear web load cell having an outer rim portion and a central hub portion, with those portions being connected by a thinner web portion, is placed to receive weight forces from a supported tank onto an upper surface of the outer rim, these forces being transmitted to a lower surface of the hub and from there to a supporting base, resulting in a deflection of the web as a function of tank weight. Measurements of weight values are obtained by use of strain gauges and associated circuitry.
Special problems are presented for outdoor tanks exposed to thermal expansion and wind forces. If the load cell is rigidly secured to a tank and to a supporting base, movements caused by these forces will result in false readings or toppling. Compensation for such movements is therefore required to obtain accurate results and prevent toppling.
Previously used mounting hardware for tank weighing assemblies has shown certain disadvantages. In some approaches a high precision parallel alignment of supporting legs of the tank and load cell connections has been required to obtain accurate results, and such alignment has proven difficult and expensive. Other approaches require use of cumbersome and heavy metal blocks, making installation procedures more difficult.
It is desired to provide a load cell tank weighing assembly in which the load cell and mount hardware are combined in an integrated unit of simple construction, the assembly enabling self-alignment and compensation for thermal and wind effects. A minimum height of the assembly is an additional desirable feature, along with protection from uplift forces due to wind.
The present invention is directed to a tank weighing assembly comprising a shear web load cell in the form of an annular disk, a supporting base plate, the bottom of the load cell and the top of the base plate having mating arcuate surfaces located at a central interface of these components and an axially extending connecting bolt having required dimensional and angular relationships with a wall of an aperture through which the bolt extends.
A concave arcuate surface may be provided at the bottom side of the load cell hub, this surface being adapted to receive a mating convex arcuate surface at the top of a supporting base plate. An axial aperture extends through each of the mating surfaces and into the base plate so that a connecting bolt may be inserted therein.
The connecting bolt has a countersunk head including an inclined lower portion tapering outward from the bolt circumference and defining a first angle with respect to vertical and an upper portion extending straight upward to the top of the bolt so as to avoid the presence of a sharp edge at the top. An aperture for receiving the bolt is provided at the axis of the load cell hub, the aperture having a first vertical portion shaped to receive the entire bolt head with slack remaining, a second, middle portion inclined to correspond to the first angle presented by the lower portion of the bolt head except for a required angular difference, preferably three degrees, and a third straight lower portion below the end of the first portion. The third straight portion of the aperture is sized to provide a substantial gap such as a total of xc2xc inch between the wall and the bolt circumference, thus allowing space for compensatory lateral involvement of the load cell.
The domed portion of the base plate, or washer-like upward projecting member, has a straight aperture similar to the third portion of the load cell but of slightly reduced diameter. The body of the base plate has a threaded aperture for receiving a threaded end portion of the bolt.
Load cells for use in this invention take the form of a hermetically sealed annular disk including a central hub portion and an outer rim portion connected by radially extending arms which are substantially thinner than the rim and central portions.
In operation, supporting legs, or a plate to which the legs are attached, bear down on the top surface of the outer ring of the load cell, and an upper surface of the base plate bears upward against a bottom surface of a central hub portion of the load cell. This causes the web to be deformed under the applied load to an extent proportional to the weight of the tank and its contents. Strain gauges mounted at appropriate locations are used to measure slight differences in impedence or resistance which occur when the web is deflected. Signals obtained from the strain gauges are then processed to provide weight values.
The upward projecting convex arcuate surface at the top of the base plate coupled with a concave receiving pocket at the bottom of the load cell provide a self-aligning connection which does not require precise parallel alignment of tank legs and related hardware.
Gaps provided between the bolt and its countersunk head and an adjacent wall of an aperture allow lateral movement of the load cell resulting from thermal expansion. The gap between straight portions of the bolt and the wall may extend for a distance of xe2x85x9 inch on each side, for a total distance of xc2xc inch. Inclined portions of the countersunk bolt head and aperture wall may have an angular difference of three degrees from one another and angular values such as 45 to 50 degrees from vertical.
A lowermost portion of the connecting bolt may be fixedly secured to the base plate by means such as tightening of threads. This restricts upward movement of the load cell to the small gap between the inclined bolt head and the adjacent inclined wall. Uplift forces as might be produced by a strong wind are thus controlled.
Tank weighing assemblies embodying the invention provide several distinct advantages. The overall height of this device is lower than for prior mounts, providing more stability. A high degree of accuracy is obtained by the shear web design, and the integrated load cell/tank mount is easier to install and less expensive to manufacture.
It is therefore, an object of this invention to provide a integrated tank weighing assembly including a load cell and mount hardware.
Another object is to provide such an assembly which enables compensation for tank movements due to thermal effects and wind forces.
Yet another object is to provide a tank weighing assembly that is inexpensive to manufacture and install.